This invention relates to a method of and system for estimating a time of arrival of a radio signal and in particular to location determination based on time of arrival estimates.
A known method of location determination consists in determining a distance between an object and a number of fixed positions. For example, in a cellular radio communication system such as the Global System for Mobile communication (GSM), the location of a subscriber unit can be determined by estimating the range from the subscriber unit to a number of fixed basestations with known locations. If the location must be determined in three dimensions, the range between the subscriber unit and three base stations must be determined. This is known as triangulation.
It is known to determine the range between the base stations from the time of arrival of signals transmitted between the subscriber unit and the base station. For example, if a signal is transmitted by a base station to the subscriber unit, the time of arrival will depend on the propagation delay which is proportional to the distance travelled by the radio signal. If the time of transmission is known the relative time of arrival can be calculated and this will correspond to the range between the base station and the subscriber unit. Typically, the subscriber units are not synchronised with the base stations, and the location is determined from the difference in the time of arrival of signals from different base station. In this case signals from at least one additional base stations must be received by the subscriber unit. The same general principle applies when location determination is based on transmitting signals from the subscriber unit to the base stations.
In for example cellular communication systems such as GSM, radio propagation is characterised by typically being along several different paths as shown in FIG. 1. These different paths can include reflections caused by various objects and as a result the paths generally have different path lengths. In FIG. 1, a base station 101 transmits to a subscriber unit 103. In the example, the propagation is such that the transmitted signal reaches the subscriber unit 103 through a direct path 105 and through a longer path 107 which results from the transmitted signal reflecting of a building 109. As a consequence of the different path lengths the received radio signal will often consist of a number of signal components being delayed versions of the same signal. In the example of FIG. 1 the two path lengths will result in two versions of the same signal being received and thus result in a propagation channel as illustrated in FIG. 2 where the first received signal component 201 corresponds to the signal received along path 105 and the second received signal component 203 corresponds to the signal received along path 107.
Each of the paths 105,107 will comprise a number of slightly different paths with slightly different path lengths and the constructive or destructive interference between versions of the signal travelling along paths with slightly different path lengths will introduce fading on each of the received signal components 201,203. The number, amplitude and delays of the individual signal components depend on the exact propagation paths and will thus vary when the environment changes or the subscriber unit moves. The phenomenon is known as multipath propagation.
The existence of multipath propagation is a significant error source when performing location measurements based on time of arrival measurements. It is therefore desirable to develop a system which mitigates the effect of multipath propagation on time of arrival determination.
The invention seeks to provide a method of and system for estimating an average time of arrival of a radio signal which mitigates the error caused by noise, interference or multipath propagation.
According to a first aspect of the present invention, there is provided a method of estimating an average time of arrival of a radio signal comprising the steps of:
transmitting a plurality of radio signals from a transmitter to a receiver through a propagation channel; estimating a time of arrival for each of a plurality of the radio signals received at the receiver; selecting a subset of the plurality of the radio signals received at the receiver in response to the time of arrival estimates; generating an averaged propagation channel estimate from the subset of radio signals; and estimating an averaged time of arrival from the averaged propagation channel estimate.
Preferably the subset consists of the radio signals having an estimated time of arrival between a lower percentile and an upper percentile of the time of arrival estimates of the plurality of the radio signals received at the receiver or alternatively the subset consists of a number of the radio signals having the closest time of arrival estimate to the time of arrival corresponding to a percentile of the time of arrival estimates of the plurality of the radio signals received at the receiver.
According to a feature of the invention at least one of the percentiles or the number of the signals is varied in response to at least one parameter chosen from the group of: a variance of the time of arrival estimates; a signal to noise ratio of at least one of the plurality of signals; a previous averaged propagation channel estimate; a propagation channel estimate of at least one of the plurality of signals; an accuracy of the averaged time of arrival estimate; and available computational resource.
According to a different aspect of the invention, there is provided a system for estimating an average time of arrival of a radio signal comprising: a receiver; a transmitter transmitting a plurality of radio signals to the receiver through a radio propagation channel; processor means for estimating a time of arrival for each of a plurality of the radio signals received at the receiver; means for selecting a subset of the plurality of the radio signals received at the receiver in response to the time of arrival estimates; means for generating an averaged propagation channel estimate from the subset of radio signals; and means for estimating an averaged time of arrival from the averaged propagation channel estimate.